Many data communication apparatuses, such as facsimile (FAX) machines, use a CCITT Group 3 (G3) mode of operation. For protocol communications, the G3 mode employs a High level Data Link Control (HDLC) format. Furthermore, some facsimile machines use zero bit insertion in conjunction with the G3 mode protocol communications. For the transmission and reception of image data, the G3 mode employs a compressed data format.
In the G3 mode, data may be improperly received by the receiving facsimile machine. If this occurs, the receiving data communication apparatus will not reproduce the image associated with the improperly received data. As a result, the user must repeat the transmission process in order for the receiving data communication apparatus to reproduce this image.
Data communication apparatuses and associated methods have however been developed to correct this problem. This involves switching from the G3 mode to the CCITT T.30 Annex A error correction method (ECM) mode. In the ECM mode, the image data is formatted into data frames. The receiving data communication apparatus will indicate to the transmitting data communication apparatus which frames of a data block where improperly received. Then, the transmitting data communication apparatus, without involvement by the user, will retransmit the improperly received data frames.
The ECM mode employs an HDLC data format similar to the HDLC protocol format used in the G3 mode. Furthermore, as is the case in the G3 mode, some data communication apparatuses use zero bit insertion in the ECM mode. As a result, it is relatively easy to switch from the G3 mode to the error correction mode for transmission and reception of the image data.
However, some data communication apparatuses use a Compatible mode rather than a G3 mode of operation for secured transmissions. For protocol communications, the Compatible mode has an inverted HDLC protocol format and does not employ zero bit insertion. For transmission and reception of image data, the Compatible mode has an inverted compressed data format.
As is the case in the G3 mode, data may be improperly received by the receiving data communication apparatus in the Compatible mode. However, because of the lack of similarity between the ECM mode and the Compatible mode, data communication apparatuses and associated methods have not yet been developed for switching from the Compatible mode to the ECM mode of operation.